INCORPORATION OF BACTERIAL EPIPHYTES INTO FRUIT COATINGS FOR POST-HARVEST BIOCONTROL OF CITRUS DECAY
US Department of Agriculture, Agricultural Research Service, 13801 Old Cutler Road, Miami, Florida, 33158, USA
Background and objectives
Formulations inciuding bacterial and yeast epiphytes have recently been commercialized for post-harvest biocontrol of citrus decay caused by Penicillium digitatum and P. italicum. BioSave 1000 (EcoScience Corporation, Orlando, Florida), incorporating strain ESC-10 of Pseudomonas syringae, and Aspire (Ecogen Inc., Langhorne, Pennsylvania), which incorporates strain I-182 of Candida oleophila, are formulated for application as sprays or dips after citrus fruit are washed but before the application of any fruit coating . This procedure allows the antagonist to fill niches that result from fruit wounding during processing and thereby compete with the pathogen for nutrients at these sites. Commercial citrus coatings often contain shellac for high gloss, and the high pH required to solubilize this material, frequently coupled with ethanol that improves film characteristics, is toxic to biocontrol yeast . Adapting these coatings to a lower pH, or substituting other materials for shellac, allows the yeast to be applied within the fruit coating . The fruit surface can support millions of yeast per cm2, and biocontrol can thereby be prolonged throughout six months of storage, rather than be focused within initial wounds. Application of bacterial antagonists within fruit coatings has not been tested but offers an alternative to yeast. The objectives of this research have been to evaluate the survival of bacterial epiphytes within fruit coatings and on the surface of grapefruit throughout four months of cold storage and to compare biocontrol between these bacteria and yeast.
Results and conclusions
P. syringae ESC-10, R. fluorescens CC-21, P. putida K-8, and Bacillus subtilis B-3 were separately added at 7x106 cfu/ml to previously developed shellac formulations of pH 7.0 to 7.2 that lack ethanol . Populations of pseudomonads in the liquid formulations generally remained above 106 cfu/ml throughout 24 h, but numbers of B. subtilis declined to approximately 5x103 cfu/ml. When mixed with shellac fruit coatings and applied to 'Marsh' white grapefruit held at 13°C, numbers of P. fluorescens dropped over 4 days to 103 cfu/cm2 whereas numbers of P. putida and B. subtilis declined to 102 cfu/cm2. Populations were fairly stable after the first week of cold storage. For comparison, populations of C. oleophila are maintained throughout 4 months of cold storage at levels above 105 cfu/cm2 on shellac-coated fruit and above 106 cfu/cm2 on fruit coated with sucrose ester or cellulose formulations. Aspire and BioSave formulations appear to be equally efficacious at controlling Penicillium decays of citrus resulting from initial processing wounds. Species of Candida and Pseudomonas are able to subsist on a wide range of food sources, but Candida species are generally more adaptive to temperature and persist well in cold storage. During long periods of cold storage, healthy fruit are more likely to become contaminated with spores of pathogens distributed by air circulation systems. The two commercial biocontrol products may be expected to find preferential use for citrus and other fruit based upon storage temperature and the relative importance of decay induced by processing or resulting from long-term storage.
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